1 DET4TC2 Measurement of Earth electrode resistance and soil resistivity……….

2 Introduction  Unique solution to the measurement of earth or ground electrode resistance and soil resistivity  Range of resistance 0.01 Ωto 200 K

3 Features  IP54 rated  2,3 and 4 point testing  Attached rod technique  Stake less measurement  User selectable test frequency (94,105,111 and128 Hz)  Dry cell or non-rechargeable battery  User selectable output voltage

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6 i)Potential probe resistance  Resistance between the potential stakes and it should be within range for accurate measurement  Rp limit: 100 KΩ (50 V output voltage) and 50 KΩ (25 V output voltage) ii)Current probe resistance  Resistance between the current stakes and it should be within range for accurate measurement  Rc limit: 100 KΩ (50 V output voltage) and 5 KΩ (25 V output voltage) iii)Ground noise voltage  The disturbance or interference caused by the formation of unwanted ground loop due to external device and system  Can be rejected upto 40 v peak to peak  Calculated by following procedure a) Connect the circuit as shown in fig b) set the rotary switch to the V position

7 FIG: Instrument connection for measuring ground noise voltage c) Then, ground noise voltage will be displayed by pressing TEST button

8 Principle of operation i)Two terminal resistance measurement  Required two earth points i.e. one earth electrode and another is good ground connection  DET injects an a.c. current into the system under test and measures the voltage develop When two terminal test is selected, DET instrument automatically connected C1-P1 and C2-P2

9  Procedure i) connect circuit as shown in fig ii) Set the rotary switch to 2P position iii) Set the required frequency iv) Click Test button v) Resistance will be displayed

10 ii) Three terminal resistance measurement  Fall of potential test  Accurately measures the resistance of an earth electrode  Uses auxiliary stake along with two terminal method  DET injects an ac current of known magnitude and measure voltage Here, C1 and P1 terminals are connected together at the electrode under test

11  In this case, the potential stake is moved by fixed increment in a straight line between electrode under test and current stake  In each location, the resistance is calculated by R= V/I then, a graph is plotted of resistance vs potential stake position  The resistance of the electrode under test is taken to be the point at which the curve is flattest  The earth electrode is consider to be surrounded by concentric shells of earth shown in fig. The current probe is also surrounded by earth shell but of smaller influence

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13  It is necessary to locate current probe to the far way so that influential shell do not overlap

14 iii) Three terminal resistance measurement using ART (Attached Rod technique)  Classic three terminal test has the disadvantage that electrode under test must be disconnected from the system it is supposed to protect at the event of fault  But here that system is connected  Current I splits into two paths i.e. I = I1 +I2

15  The current I2 is calculated using Iclamp  Then, Resistance is calculated by R= V/I2 or V/(I-I1)  Like previous away, graph is plotted to obtain exact electrode resistance

16 iv) Two clamp stake less resistance measurement  If electrode under test is connected to a network of other electrodes as shown in figure and unsafe to disconnect  Requires both Iclamp and Vclamp  Using Vclamp defined test voltage is injected into the system inducing current I which is measured by Iclamp

17 v) Four terminal resistivity measurement  Used for measuring resistivity of the soil  Stakes are kept as shown in figure with equal distance  As usual, resistance R is calculated  Then, soil resistivity is calculated as 2*π*A*R

18 THE END Any Queries ?? Prepared By: Er. Niroj Bahadur Bhujel